Effect of Long-term Nitrogen, Phosphorus and Potassium Fertilization of Irrigated Corn and Grain Sorghum

Long term research has shown that phosphorus and nitrogen fertilizer must be applied for optimum grain yields of irrigated corn and grain sorghum in western Kansas. The objectives of this study were to determine the optimum rate of nitrogen, phosphorous, and potassium fertilizer for irrigated corn and grain sorghum, and to determine the effect of long term nitrogen and phosphorous applications on nitrate accumulation in the soil profile. Although there are some year to year variations, the optimum nitrogen rate (with phosphorous) for corn remained relatively constant at about 160 lb N/A. Fertilizer phosphorous at 40 lb P₂O₅/A appeared to be adequate for producing optimum grain yield of corn. For grain sorghum, the optimal nitrogen rate varies considerably from year-to-year. In some years 40 lb N/A is sufficient to produce over 100 bu/A of grain sorghum. In other years, over 100 lb N/A is required to produce the same yield. Phosphorus fertilization increased grain sorghum yields much less than corn yields. Corn yields are routinely increased by 75 bu/A from phosphorous fertilization when applied with adequate nitrogen, compared to about 20 bu/A for grain sorghum. Potassium fertilization had no effect on grain sorghum yields. Phosphorous fertilization has reduced soil nitrate levels due to increased nitrogen use efficiency, reducing the likelihood of nitrate leaching into groundwater. Long term studies like this are important in developing an understanding of trends in crop response and reactions of fertilizer elements in soils over time. KS-23F